The present invention relates to a multi-beam scanning device for scanning a plurality of beams, which are emitted by a plurality of light sources, on objects to be scanned.
A scanning device employed in a monochromatic laser printer or the like, which forms an image in accordance with an electrophotographic imaging method, is typically provided with a laser diode, which is driven in accordance with an image signal to emit a modulated beam. A laser beam emitted by the laser diode, as modulated, is collimated by a collimating lens, deflected by a polygonal mirror to scan within a predetermined angular range, for example, in a horizontal direction. The scanning beam is refracted and converged by an fθ lens, and incident on an object, for example, a photoconductive drum so that the surface of the photoconductive drum is exposed to light in accordance with the image signal, thereby a latent image is formed thereon.
The latent image is developed as toner is applied. The developed image, i.e., the toner image is transferred onto a recording sheet, and fixed thereon.
A color printer or a color copier, which forms a color image in accordance with the electrophotographic imaging method, is also known.
Among various types of color printers or copiers, one employing a scanning device, which is provided with a plurality of light sources for respective color components, is known. Such a scanning device (which will be referred to as a multi-beam scanning device hereinafter) is provided with, for example, four light sources (laser diodes) respectively corresponding to yellow, magenta, cyan and black components of an image, and four fθ lenses for respective color components. Four photoconductive drums for the four color components are provided, and the laser beams emitted by the four laser diodes are incident on the four photoconductive drums through the four fθ lenses, respectively.
The above-described exposing, developing and transferring procedure of the electrophotographic method are performed for each color component, the four toner images for the four color components are overlaid in the transferring process, and finally, the fixing process is performed for fixing the overlaid toner images of all the color components on a recording sheet.
In the multi-beam scanning device, it is necessary that the distance between the photoconductive drums is relatively long. Downsizing of electrophotographic processing units, such as discharging unit, charging unit, developing unit and transferring unit is limited, and it is preferable that a space surrounding the photoconductive drum is large. Further, it is also preferable that a toner container which supplies toner to the developing unit is large. If the toner container is large, the toner is to be refilled less frequently.
On the other hand, for an optical system including the fθ lenses, it is important that an optical path length from a polygonal mirror to each photoconductive drum should be made as short as possible. If the optical path length is longer, the fθ lens should be made larger, and therefore, the entire device should be made larger.
Each beam is deflected by the polygonal mirror and incident on a photoconductive drum through an optical system including a plurality of optical elements. Due to the configuration of the optical components, a bow, i.e., a curvature of a scanning line in an auxiliary scanning direction may occur. If the characteristics of the optical systems for the four beams are different, and in particular, if the curved direction of at least one scanning line is opposite to the other scanning lines, the resultant color image may be significantly deteriorated in comparison with a case where all the bows curve in the same direction.